Diver Robb Robbins collects sea urchins from the seafloor in McMurdo Sound for an ocean acidification experiment being conducted by researchers at McMurdo Station.

Baseline study

Research on Antarctic sea urchins makes first long-term measurement of ocean pH

By Peter Rejcek, Antarctic Sun Editor

Posted April 6, 2012

Talk about a bad acid trip.

By the end of the 21st century, if humans keep pumping carbon dioxide (CO2) into the atmosphere as prodigiously as recent decades, the world’s oceans may become a very inhospitable place for shell-building organisms, starting with the polar regions.

That has drawn a team of researchers to McMurdo Station in Antarctica for the last two field seasons to gauge the effects of ocean acidification on the embryos and larvae of a purplish sea urchin called Sterechinus neumayeri.

“We didn’t know what the tolerances of marine invertebrates would be because the way they’re stereotyped is that they have a very narrow ocean temperature and chemistry range, so we wanted to explore that in the lab,” explained Gretchen Hofmann, an eco-physiologist and professor at the University of California, Santa Barbara.

“They’re a little more resilient than we thought,” she added. At least based on the first battery of experiments by the researchers, who spawn the critters in McMurdo’s laboratory aquarium for various experiments that mimic high levels of atmospheric CO2 on seawater.

Sea urchins and many other marine invertebrates, from corals to clams, use the calcium carbonate minerals of calcite or aragonite to construct their shell coverings or skeletons. This requires supersaturating concentrations of the carbonate ion in seawater.

Normally that’s not a problem. However, it turns out that oceans serve as a great sink for CO2, absorbing up to a third of it from the atmosphere. But it comes at a cost by altering the seawater chemistry and turning it more acidic. As ocean pH falls, so does the concentration of the carbonate ion.

The pH level, measured in units, is a calculation of the balance of a liquid’s acidity and alkalinity. The lower a liquid’s pH number, the higher its acidity. The worse case scenarios for future CO2 levels by the year 2100 translate to dangerously low pH levels, according to the 2007 Intergovernmental Panel on Climate Change (IPCC) report.

The spiky sea urchin found on the seafloor around Antarctica might be one of the first organisms to feel the effects of ocean acidification due to the cold polar waters in which it lives. That’s because CO2 more readily dissolves in colder water, meaning polar oceans would acidify first.

And there’s already evidence that the open ocean pH has dropped. Before the Industrial Revolution, ocean pH was nearly 8.2. It has dropped to about 8.1. At the end of the 21st century, under the “business as usual” model of burning fossil fuels at current rates, the IPCC report says the pH could plunge down to 7.8, which many researchers consider a threat to marine life.

In fact, research led by scientists at Stanford University in 2010 suggested that the world’s biggest extinction event 250 million years ago was caused by a combination of high levels of CO2 in the atmosphere and higher acidity in the oceans. The so-called Permian-Triassic extinction wiped out 90 percent of marine species and about three-quarters of land species.

However, there are still many uncertainties surrounding ocean acidification. Researchers have only really begun to test how different marine organisms may respond to higher acidity. An even bigger unknown is the exact pH of the ocean in different regions around the world. That 8.1 pH number is just a rough estimate.

She was also the lead author of a paper published late last year in the journal PLoS ONE that monitored the pH levels at 15 locations around the world, including seawater in the Antarctic, as well as temperate locations and the deep ocean.

“We were able to illustrate how parts of the world’s oceans currently have different pH, and thus how they might respond to climate changes in the future,” Hofmann had explained in a press release from UCSB.1 2Next